Wireless communications make a wide variety of flexible and mobile communications possible. In particular, wireless data communications is a rapidly developing market and area of technology. Numerous devices, such as laptop computers, personal data assistants (PDAs), and the like, can use wireless communications to connect to networks, printers, or other devices. A number of standards have been developed for wireless data communications so that any device conforming to a standard can communicate with any other device that conforms to the same standard. One example of a wireless communication standard is called Bluetooth.
Bluetooth is a standard primarily developed for short-distance wireless connections. Bluetooth can be used in place of a variety of proprietary cables currently used to connect devices. For instance, printers, desktop computers, fax machines, keyboards, joysticks, and virtually any other device can be part of a Bluetooth system. Bluetooth can also be used to form small networks of multiple systems, such as a network among a number of computers, server machines, printers, etc., located in the same general vicinity, such as in a room or office.
Controlling the power level of wireless transmissions is a major concern. Often, wireless devices are battery operated. Keeping power levels low prolongs the life of batteries. Also, when several wireless devices are operating in close proximity, a wireless transmission from one device may interfere with another wireless transmission from another device. Keeping power levels low reduces the amount of interference.
One common approach to managing transmission power levels relies on the received signal strength indicator (RSSI). RSSI indicates how strong a signal is at a receiver. If the RSSI drops below a certain level at a receiver, the receiver can send a command to the transmitter instructing the transmitter to increase the power level of the transmission.
Relying on RSSI has a number of disadvantages. For instance, RSSI is a measure of signal strength at the receiver. In order to control the power level at the transmitter, a “closed loop” is required. That is, the receiver has to send power control feedback messages to the transmitter. The transmitter does not decide on its own what circumstances warrant an increase or decrease in transmission power, which would be an “open loop” system.
Also, in order to decide when the transmission power can be increased or decreased, the RSSI level needs to be compared to some ideal, or target, RSSI level. The target RSSI level depends on the characteristics of the “channel” between the transmitter and the receiver. For instance, if there are high levels of interference between the transmitter and the receiver, the target RSSI level will probably be much higher than if there are low levels of interference. And of course, in a wireless channel, the channel characteristics can change over time as objects are moved, electrical devices are turned on and off, etc.
A number of approaches have been developed for determining a target RSSI. For instance, a predefined training sequence can be transmitted from time to time. The receiver can measure the channel characteristics based on the known training sequence. Then, the receiver can calculate an appropriate target RSSI based on the channel characteristics. Of course, training sequences can become tiresome to a user, like the training session at the beginning of a modem connection, and training sessions during a transmission to compensate for changes in channel characteristics can consume bandwidth that could otherwise be used for transmitting data.
The accuracy, or lack thereof, of RSSI measurements create another disadvantage to RSSI-based power control. RSSI measurements are typically only accurate to within 4 dB. This level of inaccuracy limits the granularity with which the transmission power level can be controlled. That is, the RSSI measurement may not accurately discern a power level fluctuation under 4 dB, so a transmission power level cannot be controlled with any accuracy at step sizes less than 4 dB. As a result, a power level may be set at as much as 4 dB higher than it needs to be, unnecessarily increasing the power consumption and unnecessarily increasing interference with other transmissions.